Hoolock hoolock and H.
leuconedys

Short phrases of bi-phasic hoots ("ow-wa"),
simple hoots, high pitched eeks, and low pitched growls. Bi-phasic hoots consist
of notes alternatingly produced during exhalation and inhalation, as in Hylobates
agilis (contra Haimoff, 1984). Short phrases are produced by either sex. Apparently
no sex-specific notes in song repertoire of this species. Female great call with
an acceleration-type climax, like H. moloch, of moderate speed, usually becoming
slower near end. Great call notes mainly bi-phasic. Male usually begins vocalising
halfway through the great call. Mated pairs typically produce duet song bouts only.

No vocal differences are known to occur between
the western hoolock (Hoolock hoolock) and the eastern hoolock (H. leuconedys).

Group song, Kunming Zoo, China, 27 Juli 1990.
Although most of the singing is provided by the subadult male "Jian-Jian"
and the adult female "Gui-Gui" (singing in adjacent cages), several other
immature hoolocks also contributed to the song bout.

Group song, Kunming Zoo, China, 27 Juli 1990.
Same recording as above, later in the song bout.